Terminal deoxynucleotidyl transferase (Tdt) is a key nuclear enzyme that is involved in the generation of diversity of T cell (TCR) and immunoglobulin B receptors (BCR). Tdt is not expressed in fetal sites of lymphocyte generation in mice, however, it is a normal component involved in the rearrangement process resulting in the production of diverse receptors in the adult. The lack of Tdt activity during early development contributes to the generation of T and B cell repertoires that are less diverse than their adult counterparts. The ontogenetic, developmental and cell cycle linked controls of Tdt expression are poorly understood The cellular and molecular aspects of Tdt activity on the development of B cells and the B cell repertoire will be studied in vivo using normal and multiple lines of Tdt transgenic mice developed in our laboratory. New monoclonal antibodies will be raised to the Tdt short (TdtS) and long forms (TdtL) of Tdt which will permit a detailed survey of expression of these forms in normal and eventually abnormal B cell development. The functional relationships of the two splice variants of Tdt short (TdtS) long (TdtL) will be analyzed and the hypothesis will be tested that the two forms perform distinct but complementary functions during B cell development, with TdtL exhibiting a dominant negative role in the activity of TdtS. Tdt activity is clearly important in the generation of diversity in both T and B cells and further elucidation of the mechanism of its action and control of expression are key to our understanding of the development of diversity in the immune system. Since it is expressed at high levels in certain lymphoid malignancies and shares some structural characteristics with the DNA modifying family of proteins such as BRCA1, 53BP1 the alternative forms of Tdt may play a role in tumorigenesis. The novel reagents we plan to make, together with new insights we obtain in these mouse models, will permit us to analyze in more detail the expression and function of Tdt in human lymphoid cells and the regulation and role of Tdt expression in leukemia.